Laserfiche WebLink
<br /> <br /> <br /> <br />24 <br />It is common to characterize silts using both drained <br />and undrained shear strengths similar to clays. For <br />drained conditions, the shear strength of silt can be <br />characterized by an effective stress friction angle (φ’) <br />with an assumed effective stress cohesion (c’) equal to <br />zero. For undrained conditions, the shear strength of <br />silts can be expressed using total stress strength <br />parameters (c, φ) or in terms of undrained strength, Su. <br />Similar to clays, there are different forms of <br />characterization that can be used for Su – for example, <br />constant Su, Su as a function of effective confining <br />stress, and Su as a function of depth. <br />Laboratory tests used to measure values of φ’ for silts <br />include the CD or CU’ triaxial shear test and the DS <br />test. Laboratory tests used to measure the undrained <br />shear strength (c, φ or Su) of silts include the UC test <br />(Su), UU or CU/CU’ triaxial shear test (c, φ or Su), or the <br />DSS test (Su). Similar to coarse-grained soils, it can be <br />difficult to obtain quality undisturbed samples of silts <br />in the field, particularly non-plastic or very low <br />plasticity silts. <br />Strength behaviors of silts have not been as widely <br />studied as those of sands or clays. As a result of this <br />general lack of research and compilation of data, very <br />few empirical correlations exist for predicting shear <br />strength values for silts. Empirical strength correlations <br />that are available for silts are often regionally specific <br />and developed with relatively limited data sets. Similar <br />to sands and clays, SPT, CPT, and shear wave field tests <br />can be used for empirical strength correlations of silts. <br />Empirical correlations using results of field tests for <br />sands can generally be applied to estimate shear <br />strengths of non-plastic silts. Shear strengths of plastic <br />silts can generally be estimated from empirical <br />correlations using results of field tests for clays. It <br />would be prudent to incorporate a level of <br />conservatism when using correlations for silts. <br />Shear Strengths by Federal Agency <br />Various government agencies have identified shear <br />strength envelopes to be used for design loading <br />conditions. Therefore, stability analyses performed for <br />these agencies should utilize their specific strength <br />characterization criteria. Reference [1] provides a <br />useful summary of then current guidance provided by <br />various federal agencies such as: United States Army <br />Corps of Engineers; Bureau of Reclamation; United <br />States Department of Agriculture, National Resources <br />Conservation Service; and Federal Energy Regulatory <br />Commission. Guidance documents for each agency <br />should be referenced for any updates. <br />Conclusion <br />Slope stability analyses of embankment dams are <br />highly dependent on shear strength parameters <br />assigned to the embankment and foundation soils. It is <br />therefore very important to perform a detailed shear <br />strength characterization of the various soils to obtain <br />meaningful slope stability results. Shear strength <br />characterization requires both knowledge and <br />experience to select and develop appropriate <br />parameters for various embankment and foundation <br />soils. Shear strengths of soils vary depending on the <br />loading conditions needing to be analyzed, and the <br />variations have a significant impact on slope stability <br />calculations. Careful shear strength characterization is <br />therefore an imperative, in most cases the most <br />important, component of slope stability analyses for <br />embankment dams. <br />Useful References <br />[1] Strength of Materials for Embankment Dams, United States Society on <br />Dams, February 2007. <br />[2] Soil Strength and Slope Stability, J. Michael Duncan, Stephen G. <br />Wright, and Thomas L. Brandon, 2nd Edition, 2014. <br />[3] Slope Stability during Rapid Drawdown, James M. Duncan, Stephen G. <br />Wright, and Kai S. Wong, 1992. <br />[4] Rapid Drawdown Analysis – What is An Analyst to Do? John W. France <br />and Christina J.C. Winckler, 2013. <br />[5] Fundamentals of Soil Behavior, James K. Mitchell and Kenichi Soga, 3rd <br />Edition, 2005. <br />[6] Engineering Field Manual, Chapter 4 - Elementary Soil Engineering, <br />U.S. Department of Agriculture, Natural Resources Conservation <br />Service (NRCS), July 1984 (Fourth Printing) <br />[7] Earth Manual, Part 1, U.S. Department of the Interior, Bureau of <br />Reclamation, 1998 (Third Edition) <br />[8] EM 1110-1-1804, Geotechnical Investigations, U.S. Army Corps of <br />Engineers, January 2001 <br />[9] EM 1110-1-1802, Geophysical Exploration for Engineering and <br />Environmental Investigations, Department of the Army, U.S. Army <br />Corps of Engineers, 31 August 1995 <br />[10] FHWA NHI-06-088, Soils and Foundations, Reference Manual – <br />Volume 1, U.S. Department of Transportation, Federal Highway <br />Administration, December 2006 <br />[11] Manual on Estimating Soil Properties for Foundation Design, EPRI EL- <br />6800, Electric Power Research Institute (EPRI), August 1990. <br />[12] New design procedure for stability of soft clays, Ladd, CC. and Foot, R. <br />(1974). ASCE Journal of the Geotechnical Engineering Division. Vol <br />100, No GT7, pp 763-786. <br />[13] Stress-deformation and strength characteristics. Ladd, CC, Foot, R, <br />Ishihara, K, Schlosser, F and Poulos, HG. (1977).International <br />conference on soil mechanics and foundation engineering, 9, <br />Proceedings, Vol. 2, pp 421- 494. Tokyo.